The cultivation of agricultural crop plants serves mainly for the production of foodstuffs for humans and animals. Monocultures in particular, which are the rule nowadays, are highly susceptible to an epidemic-like spreading of diseases. The result is markedly reduced yields. To date, the pathogenic organisms have been controlled mainly by using pesticides. Nowadays, the possibility of directly modifying the genetic disposition of a plant or pathogen is also open to man.
Resistance generally describes the ability of a plant to prevent, or at least curtail the infestation and colonization by a harmful pathogen. Different mechanisms can be discerned in the naturally occurring resistance, with which the plants fend off colonization by phytopathogenic organisms. These specific interactions between the pathogen and the host determine the course of infection (Schopfer and Brennicke (1999) Pflanzenphysiologie, Springer Verlag, Berlin-Heidelberg, Germany).
With regard to the race specific resistance, also called host resistance, a differentiation is made between compatible and incompatible interactions. In the compatible interaction, an interaction occurs between a virulent pathogen and a susceptible plant. The pathogen survives, and may build up reproduction structures, while the host mostly dies off. An incompatible interaction occurs on the other hand when the pathogen infects the plant but is inhibited in its growth before or after weak development of symptoms (mostly by the presence of R genes of the nucleotide-binding site-leucine-rich repeat (NBS-LRR) family, see below). In the latter case, the plant is resistant to the respective pathogen (Schopfer and Brennicke, vide supra). However, this type of resistance is specific for a certain strain or pathogen.
In both compatible and incompatible interactions a defensive and specific reaction of the host to the pathogen occurs. In nature, however, this resistance is often overcome because of the rapid evolutionary development of new virulent races of the pathogens (Neu et al. (2003) American Cytopathol. Society, MPMI 16 No. 7: 626-633).
Most pathogens are plant-species specific. This means that a pathogen can induce a disease in a certain plant species, but not in other plant species (Heath (2002) Can. J. Plant Pathol. 24: 259-264). The resistance against a pathogen in certain plant species is called non-host resistance. The non-host resistance offers strong, broad, and permanent protection from phytopathogens. Genes providing non-host resistance provide the opportunity of a strong, broad and permanent protection against certain diseases in non-host plants. In particular, such a resistance works for different strains of the pathogen.
Fungi are distributed worldwide. Approximately 100 000 different fungal species are known to date. Thereof rusts are of great importance. They can have a complicated development cycle with up to five different spore stages (spermatium, aecidiospore, uredospore, teleutospore and basidiospore).
During the infection of plants by pathogenic fungi, different phases are usually observed. The first phases of the interaction between phytopathogenic fungi and their potential host plants are decisive for the colonization of the plant by the fungus. During the first stage of the infection, the spores become attached to the surface of the plants, germinate, and the fungus penetrates the plant. Fungi may penetrate the plant via existing ports such as stomata, lenticels, hydatodes and wounds, or else they penetrate the plant epidermis directly as the result of the mechanical force and with the aid of cell-wall-digesting enzymes. Specific infection structures are developed for penetration of the plant.
Immediately after recognition of a potential pathogen the plant starts to elicit defense reactions. Mostly the presence of the pathogen is sensed via so called PAMP receptors, a class of trans-membrane receptor like kinases recognizing conserved pathogen associated molecules (e.g. flagellin or chitin). Receptor-like kinases (RLKs) are signaling proteins that feature an extracellular domain connected via a transmembrane domain to a cytoplasmic kinase. This architecture indicates that RLKs perceive external signals, transducing them into the cell. In plants, RLKs were first implicated in the regulation of development, in pathogen responses, and in recognition events. (Santiago A Morillo and Frans E Tax (2006) Functional analysis of receptor-like kinases in monocots and dicots. Current Opinion in Plant Biology 9:460-469).
Downstream of the PAMP receptors, the phytohormones salicylic acid (SA), jasmonate (JA) and ethylene (ET) play a critical role in the regulation of the different defense reactions. Depending on the ratio of the different phytohormones, different defense reactions are elicited by the host cell. Generally SA dependent defense is linked with resistance against biotrophic pathogens, whereas JA/ET dependent defense reactions are active against necrotrophic pathogens (and insects).
In addition to the localized defense responses, plants that were attacked locally by a pathogen induce a “whole-plant” long lasting systemic defence response called systemic acquired resistance (SAR). SAR is associated with the induction of a wide range of genes (so called PR or “pathogenesis-related” genes), a burst of the reactive oxygen species (ROS), ethylene production and the accumulation of salicylic acid (SA).
Lee and Hwang (Planta 2005, Vol. 221: 790-800) showed that a gene called SAR8.2 (also known as CASAR for “Capsicum annuum SAR”) accumulates in systemic pepper leaves as a result of a bacterial and fungal pathogen infection, abiotic elicitors, and by environmental stresses such as drought, salt, and low temperatures. The ectopic expression of CASAR in Arabidopsis leads to a constitutive expression of the PR-genes including AtPR-1, AtPR-4 and AtPR-5. Additionally the CASAR overexpression in Arabidopsis enhanced the resistance against infections by Pseudomonas syringae pv. tomato, Fusarium oxysporum f.sp. matthiolae or Botrytis cinerea (Lee and Hwang, Plant Molecular Biology (2006) 61:95-109). In addition Lee and Hwang (Plant Molecular Biology (2006) 61:95-109) showed that purified recombinant CASAR protein and crude protein extracts of the transgenic plants exhibited antifungal activity against some phytopathogenic fungi.
Lee and Hwang (Plant Molecular Biology (2006) 61:95-109) do not demonstrate enhanced resistance or antifungal activity against heminectrotrophic fungi, in particular against fungal pathogens of the order Pucciniales (rust). This specific group of fungal pathogens is characterized by a unique life-cycle.
For instance, the soybean rust Phakopsora pachyrhizi directly penetrates the plant epidermis. After crossing the epidermal cell, the fungus reaches the intercellular space of the mesophyll, where the fungus starts to spread through the leaves. To acquire nutrients the fungus penetrates mesophyll cells and develops haustoria inside the mesophyll cell. During the penetration process the plasmamembrane of the penetrated mesophyll cell stays intact. Therefore the soybean rust fungus establishes a biotrophic interaction with soybean.
The biotrophic phytopathogenic fungi, such as soybean rust and all other rust fungi, depend for their nutrition on the metabolism of living cells of the plants. This type of fungi belong to the group of biotrophic fungi, like other rust fungi, powdery mildew fungi or oomycete pathogens like the genus Phytophthora or Peronospora The necrotrophic phytopathogenic fungi depend for their nutrition on dead cells of the plants, e.g. species from the genus Fusarium, Rhizoctonia or Mycospaerella. Soybean rust has occupied an intermediate position, since it penetrates the epidermis directly, whereupon the penetrated cell becomes necrotic.
After the penetration, the fungus changes over to an obligatory-biotrophic lifestyle. The subgroup of the biotrophic fungal pathogens which follows essentially such an infection strategy is heminecrotrophic. In contrast to a heminecrotrophic pathogen, a hemibiotrophic pathogen lives for a short period of time in a biotrophic manner and subsequently starts killing the host cell and/or host organism, i.e., changes for the rest of its life-cycle to a necrotrophic life-style.
Soybean rust has become increasingly important in recent times. The disease may be caused by the biotrophic rusts Phakopsora pachyrhizi and Phakopsora meibomiae. They belong to the class Basidiomycota, order Pucciniales (rust), previously also known as Uredinales, family Phakopsoraceae. Both rusts infect a wide spectrum of leguminosic host plants. P. pachyrhizi, also referred to as Asian rust, is the more aggressive pathogen on soy (Glycine max), and is therefore, at least currently, of great importance for agriculture. P. pachyrhizi can be found in nearly all tropical and subtropical soy growing regions of the world. P. pachyrhizi is capable of infecting 31 species from 17 families of the Leguminosae under natural conditions and is capable of growing on further 60 species under controlled conditions (Sinclair et al. (eds.), Proceedings of the rust workshop (1995), National SoyaResearch Laboratory, Publication No. 1 (1996); Rytter J. L. et al., Plant Dis. 87, 818 (1984)). P. meibomiae has been found in the Caribbean Basin and in Puerto Rico, and has not caused substantial damage as yet.
P. pachyrhizi can currently be controlled in the field only by means of fungicides. Soy plants with resistance to the entire spectrum of the isolates are not available. When searching for resistant soybean accessions, six dominant R-genes of the NBS-LRR family, named Rpp1-5 and Rpp? (Hyuuga), which mediate resistance of soy to P. pachyrhizi, were discovered by screening thousands of soybean varieties. As the R-genes are derived from a host (soybean), the resistance was lost rapidly, as P. pachyrhizi develops new virulent races.
In recent years, fungal diseases, e.g. soybean rust, has gained in importance as pest in agricultural production. There was therefore a demand in the prior art for developing methods to control fungi and to provide fungal resistant plants.
Much research has been performed on the field of powdery and downy mildew infecting the epidermal layer of plants. However, the problem to cope with soybean rust which infects the mesophyll remains unsolved.
The object of the present invention is inter alia to provide a method of increasing resistance against fungal pathogens, preferably rust pathogens (i.e., fungal pathogens of the order Pucciniales), preferably against fungal pathogens of the family Phakopsoraceae, more preferably against fungal pathogens of the genus Phakopsora, most preferably against Phakopsora pachyrhizi and Phakopsora meibomiae, also known as soybean rust.
Surprisingly, we found that fungal pathogens, in particular of the order Pucciniales, in particular of the family Phakopsoraceae, for example soybean rust, can be controlled by increasing the expression of a CASAR gene.
The present invention therefore provides a method of increasing resistance against fungal pathogens, preferably against rust pathogens (i.e., fungal pathogens of the order Pucciniales), preferably fungal pathogens of the family Phakopsoraceae, more preferably against fungal pathogens of the genus Phakopsora, most preferably against Phakopsora pachyrhizi and Phakopsora meibomiae, also known as soybean rust, in transgenic plants, transgenic plant parts, or transgenic plant cells by overexpressing one or more CASAR nucleic acids.
A further object is to provide transgenic plants resistant against fungal pathogens, preferably rust pathogens (i.e., fungal pathogens of the order Pucciniales), preferably of the family Phakopsoraceae, more preferably against fungal pathogens of the genus Phakopsora, most preferably against Phakopsora pachyrhizi and Phakopsora meibomiae, also known as soybean rust, a method for producing such plants as well as a vector construct useful for the above methods.
Therefore, the present invention also refers to a recombinant vector construct and a transgenic plant, transgenic plant part, or transgenic plant cell comprising an exogenous CASAR nucleic acid. Furthermore, a method for the production of a transgenic plant, transgenic plant part or transgenic plant cell using the nucleic acid of the present invention is claimed herein. In addition, the use of a nucleic acid or the recombinant vector of the present invention for the transformation of a plant, plant part, or plant cell is claimed herein.
The objects of the present invention, as outlined above, are achieved by the subject-matter of the main claims. Preferred embodiments of the invention are defined by the subject matter of the dependent claims.